Knudsen number

About: Knudsen number is a research topic. Over the lifetime, 5052 publications have been published within this topic receiving 104278 citations.

High-Altitude Capsule Aerodynamics with Real Gas Effects

Abstract: Re-entry capsule aerodynamics within a wide range of angles of attack and flight altitudes are examined by the direct simulation Monte Carlo method. The local bridging method is verified by comparison with results of simulations. Capsule stability is analyzed for flight altitudes from 130 km down to 85 km. Comparison between computed and free flight data shows a good agreement. A qualitative change of heat transfer coefficient behavior for different angles of attack during the descent is revealed. The influence of chemical reactions on aerodynamics and flowfields at 85 km is shown to be significant. For a flow simulation in the near-continuum regime, a parallel version of the direct simulation code, with static and dynamic load balancing techniques, is used. An efficiency of about 80% is obtained for 256 processors using dynamic load balancing. Nomenclature CA = axial force normalized by p^U^S/2 CH - heat transfer normalized by p^U^S/ Cm - pitching moment normalized by p^ CN = normal force coefficient normalized by p^U^S/2 Ck = local coefficient Q cont = continuum coefficient ckjm = free molecular coefficient Fb = bridging function H = altitude, km Kn = Knudsen number KnotOC = Knudsen number based on /z (), TO, and p^ L = characteristic length, m NI = number of molecules in cell / Nm = average number of simulated molecules in the computational domain Wproc = number of processors S = characteristic size, m2 TO = stagnation temperature, K kaic = calculation time rcom = communication time Adic = synchronizatio n time fun = total operation time f/oo = freestream velocity, m/s a = angle of attack, deg £/ = volume of interaction region in cell /, m3 jjio = stagnation viscosity, kg/m-s v"1 = majorant frequency, s"1 Poo = freestream density, kg/m3

The self-preserving size distribution theory. I. Effects of the Knudsen number on aerosol agglomerate growth.

Abstract: Gas-phase synthesis of fine solid particles leads to fractal-like structures whose transport and light scattering properties differ from those of their spherical counterparts. Self-preserving size distribution theory provides a useful methodology for analyzing the asymptotic behavior of such systems. Apparent inconsistencies in previous treatments of the self-preserving size distributions in the free molecule regime are resolved. Integro-differential equations for fractal-like particles in the continuum and near continuum regimes are derived and used to calculate the self-preserving and quasi-self-preserving size distributions for agglomerates formed by Brownian coagulation. The results for the limiting case (the continuum regime) were compared with the results of other authors. For these cases the finite difference method was in good in agreement with previous calculations in the continuum regime. A new analysis of aerosol agglomeration for the entire Knudsen number range was developed and compared with a monodisperse model; Higher agglomeration rates were found for lower fractal dimensions, as expected from previous studies. Effects of fractal dimension, pressure, volume loading and temperature on agglomerate growth were investigated. The agglomeration rate can be reduced by decreasing volumetric loading or by increasing the pressure. In laminar flow, an increase in pressure can be used to control particle growth and polydispersity. For D(f)=2, an increase in pressure from 1 to 4 bar reduces the collision radius by about 30%. Varying the temperature has a much smaller effect on agglomerate coagulation.

Higher order slip according to the linearized Boltzmann equation with general boundary conditions

Abstract: In the present paper, we provide an analytical expression for the first- and secondorder velocity slip coefficients by means of a variational technique that applies to the integrodifferential form of the Boltzmann equation based on the true linearized collision operator and the Cercignani–Lampis scattering kernel of the gas–surface interaction. The polynomial form of the Knudsen number obtained for the Poiseuille mass flow rate and the values of the velocity slip coefficients are analysed in the frame of potential applications of the lattice Boltzmann methods in simulations of microscale flows.

Kinetic simulations of thermal escape from a single component atmosphere

Abstract: The one-dimensional steady-state expansion of a monatomic gas from a spherical source in a gravity field is studied by the direct simulation Monte Carlo method. Collisions between molecules are described by the hard sphere model, the distribution of gas molecules leaving the source surface is assumed to be Maxwellian, and no heat is directly deposited in the simulation region. The flow structure and the escape rate (number flux of molecules escaping the atmosphere) are analyzed for the source Jeans parameter λ0 (ratio of the gravitational energy to thermal energy of the molecules) and Knudsen number Kn0 (ratio of the mean free path to the source radius) ranging from 0 to 15 and from 0.0001 to ∞, respectively. In the collisionless regime, flows are analyzed for λ0=0-100 and analytical equations are obtained for asymptotic values of gas parameters that are found to be non-monotonic functions of λ0. For collisional flows, simulations predict the transition in the nature of atmospheric loss from escape on a m...